to always be a problem with the underdeck tanks as they usually let water in and petrol out. On mine the problem was due to two things. Firstly the fuel sender units are made of different metal to the tanks and so corrode and leak. Secondly the fuel pick up in the tanks is not at the bottom so even if you run the tanks "dry" there is still about 20 litres of fuel / water left in the tank which can only be removed through the sender fixing.
ok its a specific boat...but it ain't always good under the boardwalk and if it goes wrong its a pain.
On a long run, with under deck or in the consol tanks, I would by preference also carry the portable tanks, and run on them first. Its so much easier when you get to a harbour/marina without a petrol pump. And up in God's own country that means all of them. Otherwise you are carrying jerry cans and I may as well run with sommat I can simply plug into the system and turn the changeover valve to them rather than try and pour fuel in whatever the conditions might be at the time.